This invention relates to a fuel metering system having improved ability to handle transient fuel metering modes of operation. More particularly, it relates to a fuel metering system for an internal combustion engine wherein the fuel control system of the engine is better enabled, as compared to the prior art, to handle the transient conditions that occur during engine acclerations, decelerations (negative acceleration) and other conditions that cause fluctuations to occur on a temporary basis in the flow of fuel from the engine's primary fuel metering apparatus to its combustion chamber or chambers.
In internal combustion engines, the rate at which fuel is metered to the engine varies during engine operation. Changes in engine load cause the engine's fuel metering apparatus to increase or to decrease the rate at which fuel is metered to the engine. As a result, the engine must change from a first state, where engine operation and fuel flow rate is quite stable, to a second state, where these conditions again become stable. The conditions in between the stable states are of a transient character in that the rate of fuel flow varies continuously and can produce undesirable air/fuel ratios. For example, with carburetion or other central location of the fuel metering apparatus, there is an intake manifold passage that the vaporized or atomized fuel must traverse in order to reach the engine's combustion chamber or chambers. At a given engine load, prior art fuel control systems under transient engine operation are unable to maintain precise air/fuel ratios until the conditions in the engine's intake passages have stabilized. Sudden accelerations cause an increase in the rate at which liquid fuel is deposited on the walls of the intake passages (wall wetting), and sudden decelerations produce a lessened rate of deposition. The reason for this has to do with the changing vapor pressures. The higher the vapor pressure, the more the fuel tends to accumulate on the walls of the intake passages. Vapor pressure is a partial pressure, and the major contributor to pressure in the intake passage is air. The air pressure in the intake passages in general is below atmospheric, unless the usual throttle valve is fully open, during engine operation.
While the wall-wetting changes, the amount of fuel metered by the fuel metering apparatus on the engine is not the amount of fuel that actually reaches the engine's combustion chambers within the charge transport time (air/fuel delivery time) applicable to the particular engine speed and load conditions at the time. The engine speed and load under stable engine operating conditions are the factors primarily determinative of the transport time of the air/fuel mixture from the fuel metering apparatus to the engine's respective combustion chambers. This applies to both central point fuel metering and multipoint fuel metering systems. Central point fuel systems include both the conventional carburetion system and the recently developed central point fuel injection system that has two electromagnetic fuel injectors positioned in a throttle body (air valve) to inject fuel into the incoming airstream. The multipoint system is exemplified by electronic fuel injection systems that provide an electromagnetic fuel injector for each of the engine's combustion chambers, with each injector injecting fuel into the intake passage immediately upstream of the intake valve for the associated combustion chamber.